1. Field of the Invention
This invention relates to reducing the phytotoxicity of conventional horticultural petroleum spray oils. More specifically, the invention relates to the incorporation in such oils of auto-oxidation inhibitors which are themselves non-phytotoxic non-carcinogenic and safe for use on crop. In particular, the invention is directed to preventing oxidation of spray oils which are exposed to strong sunlight over a protracted length of time.
Petroleum oils are used in agriculture as carriers or solvents for spraying pesticides (fungicides, insecticides, ovicides, etc.), herbicides, micronutrients and various types of chemical adjuvants. They are also used as agricultural spray oils in their own right, i.e., without additives, because of their natural herbicidal and pesticidal properties. They control a wide range of pests, for examples, Sigatoka disease in banana, earworm in sweet corn, Cercospora in sugar beet, and mites, aphids, scale insects in decideous fruit trees, citrus and ornamentals. The main advantage of petroleum spray oils are their relative cheapness, their low health hazard and the apparent inability of fungi, insects and mites to develop strains resistant to them.
The properties required for an oil to perform efficiently as a carrier do not necessarily conflict with those required for an oil to act as a natural herbicidal or pesticidal spray oil. Indeed many commercial spray oils perform both functions. Present day conventional petroleum spray oils are derived from light lube oil distillates.
The horticultural spray oils have a gravity API of 31-36, a viscosity at 100.degree. F. of 81 to 87 Saybolt Universal Seconds and a boiling range between 600.degree. and 775.degree. F. An analysis of four preferred oils samples is given in Table I. For normal applications highly refined carrier oils, predominantly paraffinic, having a minimum API gravity of 27, viscosity at 100.degree. F. of between 55 and 100 Saybolt Universal Seconds, boiling range of 600.degree.-775.degree. F. and minimum unsulphonated residue of 85%, are generally used. A lighter oil with API gravity around 50 and boiling point range 85.degree.-600.degree. F. belonging to the general groups of paraffins, isoparaffins and/or naphthenes is desirable for use with light-weight ultra low volume sprayers.
TABLE I ______________________________________ ANALYSES OF CARRIER OILS USED Carrier Oil Samples RS- RS- RS- RS- 1200/67 932/67 175/60 370/71 ______________________________________ Specific Gravity at 60/60.degree. F (ASTM D-1298) 0.8493 0.8487 0.8523 0.8492 Gravity, .degree. API (ASTM D-1250) 35.1 35.2 34.5 35.1 Flash Point (COC), .degree. F (ASTM D-92) 395 395 405 395 Viscosity at 100.degree. F SUS (ASTM D-445/2161) 84.2 83.3 86.5 81.6 Color (ASTM D-1500) &lt;0.5 0.5 0.5 0.5 Pour Point, .degree. F (ASTM D-97) 5 5 20 5 Ash, wt. % Trace Trace 0.003 0.005 Corrosion Copper Strip 3 hrs. at 212.degree. F (ASTM D-130) Pass Pass 1A 1A Neutralization Number mg. KOH/g 0.04 0.09 0.054 0.044 Unsulfonated Residue, vol. % (ASTM D-483) 94.2 94 92.8 91.4 Distillation, .degree. F IBP 637 649 649 610 10% Recovery 675 685 674 671 90% Recovery 729 730 729 732 FBP 745 -- 750 756 ______________________________________
Physical data for a typical spray oil are given in Table I. There is a definite relationship between the unsaturates (aliphatic or cyclic hydrocarbons that have one or more active double or triple bonds) content of petroleum oils and phytotoxicity or leaf burn. It has been established that the aromatic hydrocarbon components are indirectly responsible for leaf injury. Thus R. P. Tucker (Ind. Eng.Chem., 1936, 28, 458) has shown that the aromatic hydrocarbons are not toxic to the foilage of plants in a chemical sense until they are oxidized to oil soluble asphaltogenic acids. Saturated hydrocarbon components are also oxidized to acid material. However, the rate of oxidation of the saturates at ordinary temperatures is so slow that they are chemically inert towards foliage. The real causes of phytotoxicity have yet to be determined; it is believed that there is both a chemical action and a physical one. The tendency of the unsaturated hydrocarbons to oxidize to asphaltogenic acids is undoubtedly the principal non-physical factor involved in phytotoxicity. It is a fact that conventional spray oils low in aromatic hydrocarbons are relatively non-phytotoxic.
In understanding the present invention it should be remembered that the unsulfonated residue (UR) content of a conventional petroleum spray oil is a measure of the degree of refinement, ie. the absence of aromatics which directly or indirectly cause leaf burn and fruit blotch. Petroleum spray oils are refined by extraction of the aromatics with solvents, e.g., furfural, or by reaction with sulphuric acid. The UR content of an oil is determined by ASTM D 483 which gives an approximate content of the saturated hydrocarbons present by measuring the percent unreactive to 37N sulphuric acid. Light mineral oils with UR levels in the range 70-100 are equally effective in disease control. However, the degree of leaf damage is related to the UR level. In a similar boiling range, oils with the highest UR level cause the least damage. Location may also be a factor. Thus citrus trees are not so adversely affected by an oil of low UR level in more humid regions. A relatively safe level of UR is 92% indicating the percentage of non-aromatic hydrocarbon components in the oil.
The rate of autoxidation of oils containing an appreciable amount of unsaturated hydrocarbons and aromatics, i.e., relatively low UR level, is very slow in the absence of light but sunlight activates the oxidation to a marked degree forming the toxic asphaltogenic acids. The amount of acid injurious to foliage is relatively small.